Terminal electron acceptor in noncyclic photophosphorylation In the noncyclic (linear) electron flow of the light reactions, electrons originating from water are ultimately accepted by which molecule?
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ANADP+
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BChlorophyll a
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CCarbon dioxide
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DChlorophyll b
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EUbiquinone in mitochondria
Answer
Correct Answer: NADP+
Explanation
Introduction / Context:Noncyclic photophosphorylation transfers electrons from water to NADP+, generating both ATP and reducing power (NADPH) for the Calvin cycle. Identifying the terminal electron acceptor is key to understanding the function of photosystem I and downstream reductive metabolism.
Given Data / Assumptions:
- Photosystem II oxidizes water, releasing electrons, protons, and O2.
- Electrons move through cytochrome b6f and plastocyanin to photosystem I.
- Ferredoxin NADP+ reductase catalyzes the final reduction step.
Concept / Approach:
The linear pathway is water → PSII → plastoquinone → cytochrome b6f → plastocyanin → PSI → ferredoxin → NADP+. The last acceptor is NADP+, which becomes NADPH. This reductant powers CO2 fixation and reduction in the Calvin cycle.
Step-by-Step Solution:
Track electrons from water via PSII to PSI.After PSI excitation, ferredoxin carries electrons to the stromal enzyme FNR.FNR reduces NADP+ to NADPH using two electrons and a proton.Therefore, NADP+ is the terminal acceptor of noncyclic flow.Verification / Alternative check:
Spectroscopic and biochemical assays show accumulation of NADPH under illumination with active PSI and FNR, confirming NADP+ as the acceptor.
Why Other Options Are Wrong:
Chlorophyll a and b are photopigments, not final electron sinks. CO2 is reduced in the Calvin cycle by NADPH, not directly by the electron transport chain. Ubiquinone is a mitochondrial carrier, not a chloroplast component.
Common Pitfalls:
Confusing the terminal acceptor of chloroplast electron flow (NADP+) with that of mitochondria (O2).
Final Answer:
NADP+